The present invention relates to devices for grinding and shredding large, rigid objects and, more particularly, to devices utilizing large screw augers for grinding and shredding such material.
Many devices are known which are capable of grinding, shredding and otherwise reducing the size of scrap material such as wooden pallets, wooden crates, fifty-five-gallon oil drums of waste material such as concrete, railroad ties, and the like. For example, Koenig U.S. Pat. No. 4,253,615 discloses a pallet auger which includes a grinding chamber within which is mounted a single screw having a tapered flight and which extends from a substantially vertical rear wall into a discharge conduit extending through the front wall of the grinding chamber. The screw flight includes teeth which project radially from the periphery of the flight and mesh with fixed breaker bars positioned on the side walls and floor of the grinding chamber, which together form a continuous, arcuate surface sloped to provide a close clearance with the tapered flight.
A different design is disclosed in Wexell, et al. U.S. Pat. No. 4,632,317. That device discloses a multiple screw grinding device having an open-bottomed grinding chamber and a plurality of auger screws, each having a non-tapered flight and being offset with respect to each other.
A disadvantage with both types of devices is that there is no mechanism for controlling the ultimate size of the material which is ground by the auger screws. With the Koenig device, some size control can be achieved by restricting the flow of ground material through the exit conduit of the grinding chamber. This "back pressure" allows the end of the screw flight, which includes a radially-extending edge, to perform a shredding action upon a plug of material retained within the exit opening. In contrast, the Wexell et al. device is designed to be a "single pass" device in which the ultimate size of material shredded is a function of the spacing between the slighted grinding screws which are positionable relative to each other.
Another disadvantage existing with the Wexell et al. device is that the non-tapered screw flights present a level and even surface to objects such as pallets and large crates so that the broad faces of those objects, when fed downwardly onto the screws, ride or bridge the screw flights. It is then necessary for an operator to manually press the material into the cutting edges of the screw flights, a labor-intensive and often dangerous procedure.
Accordingly, there is a need for a device which accepts and grinds large objects such as pallets, crates, oil drums and the like which has the capability of controlling the ultimate size of the pieces of the shredded material. There is also a need for a device for grinding and shredding large objects which is especially suited to accept objects having large surfaces and which prevents a bridging or riding upon the grinding elements.